Add support for ```BGMFileExtension```

Add support for ```BGMFileExtension``` in the API to define a list of BGM extensions to load.

Until now, BGM had to be ```.mp3``` files, as it's the default audio format used in the game.
```BGMFileExtension``` allow you to define a list of extensions, fetch the list until it found a format the browser can read. Then all request to the BGM will be redirect to the new file extension.

For now, it only support ```.ogg``` file, an audio file lighter than the ```.mp3```.